Recently, along with the development of digital processing techniques for video signals, household VCRs have come onto the market that are designed to provide higher resolution and better quality images. In an improved VHS system generally referred to as super-VHS (S-VHS), the full bandwidth of NTSC television signal is recorded on a magnetic recording medium of a limited bandwidth in the video tape cassette. Then, an image with improved quality is reproduced, providing high resolution.
The specification and drawing of allowed U.S. patent application Ser. No. 07/569,029 filed by the inventors and their co-workers on 17 Aug. 1990; entitled "AN IMPROVED VIDEO SIGNAL RECORDING SYSTEM"; and assigned to Samsung Electronics Co., Ltd., (now U.S. Pat. No. 5,113,262) is incorporated herein by reference. This system was developed to provide a higher-resolution home VCR that unlike S-VHS home VCRs is compatible with the standard VHS system.
The video signal recording and playback system that is described in U.S. Pat. No. 5,173,262 follows in some respects the standard VHS system procedure for recording and playing chroma information in the color-under format. The standard VHS system procedure for playing back chroma information recorded in the color-under format includes measures as described in the next paragraph, which prevent time-base error (TBE) in the video signals from introducing color errors. For example, TBE occurs in the video signals to be recorded when they are supplied from another video recorder owing to speed variations of the tape transport used for playback, to tape vibration, etc. During recording, further TBE arises owing to speed variations of the tape transport used for recording, to tape vibration, etc. During subsequent playback, still further TBE arises owing to speed variations of the tape transport used for playback, to tape vibration, etc.
During recording, separated chrominance sidebands with a suppressed 3.58 MHz carrier are heterodyned with 4.21 MHz oscillations to generate complex amplitude-modulation sidebands of a 629 kHz suppressed carrier. Intervening color bursts are heterodyned down to 629 kHz during this down-conversion procedure for generating a color-under signal. The 4.21 MHz oscillations used in the down-conversion are a so-called "nervous clocking signal" supplied from a phase-locked oscillator (PLO). This PLO includes a voltage-controlled oscillator (VCO), the frequency and phase of the VCO being electrically controlled by an error signal and synchronized to a multiple of the scan line frequency. To achieve such synchronization, the VCO is included in a corrective-feedback loop connection in which the VCO supplies its oscillations to a frequency-divider, the frequency-divider supplies a submultiple of the oscillations to the frequency and phase comparator, the frequency and phase comparator supplies its comparison results to a Loop filter that determines the speed with which the error signal can be diminished by the loop connection, and the filter response is applied to the VCO for regulating the frequency and phase of its oscillations, thereby to completing the corrective-feedback Loop that adjusts the frequency and phase of the oscillations from the VCO to diminsh the error signal. If the filter response time constant is not appreciably longer than a scan line duration, the frequency and the phase of the 4.21 MHz oscillations vary in accordance with the TBE in the horizontal synchronization. In the down-conversion to 629 kHz, the TBE in the chroma differentially combines with the TBE in the 4.21 MHz oscillations, so that the color-under carrier is essentially free of TBE. During playback, in an up-conversion procedure, stable 4.21 MHz oscillations are heterodyned with the color-under signal to regenerate color burst and chroma sidebands, the 3.58 MHz carrier of which is essentially free of TBE.
The video signal recording and playback system that is described by the inventors and their co-inventors in U.S. patent application Ser. No. 07/569,029 departs from the standard VHS system procedure for recording and playing chroma information in the color-under format, and here it is pointed out. The 4.21 MHz sinusoidal signals used in the down-converting and up-converting procedures associated with color-under signals are not generated directly using a phase-locked analog oscillator oscillating at a 4.21 Mhz frequency. Instead, a higher-frequency analog oscillator is phase-locked to the horizontal synchronizing pulses and its oscillations are counted to determine pixel scanning addresses along each scan line. The pixel scanning addresses are used in the spatio-temporal filtering of digitized luminance signals. The 4.21 Mhz frequency nervous clocking signal is then derived from the higher-frequency oscillations using a sine-wave look-up table stored in read-only memory (ROM) or is generated by heterodyning a stable oscillator with the higher-frequency oscillations or a submultiple of those oscillations as obtained by frequency division.
The playback circuitry for a video signal recording and playback system that is described in U.S. patent application Ser. No. 07/569,029 includes a time-base corrector (TBC) that corrects during playback for the time-base error (TBE) in the luminance signal. A TBC designates a device in which time-base error included in a video signal is eliminated by a memory which functions as a time-base buffer. The video signal with the TBE is written into the memory in accordance with a clock signal synchronized with the video signal and the TBE and is read out in accordance with a stable clock signal. The term "TBE" denotes the jitter introduced into a signal by the mechanical factors in a VCR recording and playback system, such as speed variations of the tape transport during recording and during playback, tape vibration, etc. Because of the TBE in the output signal from a VCR, such a signal is classified by circuitry in the VCR as being a non-standard signal. Digital signal processing techniques that require high time-base stability, in order to implement signal processing in the temporal dimension, are, the inventors believe, best discontinued when the VCR receives an input signal that exhibits sufficient jitter to be classified as a non-standard signal.
That is to say, suppose a first high-resolution home VCR, employing the frequency-folding technique to reduce the bandwidth of recorded luminance signal, plays back a video signal that could be classified as being a non-standard signal because of the substantial amount of TBE included in that signal; and suppose further that the played-back video signal is to be recorded by a second high-resolution home VCR, employing the frequency-folding technique to reduce the bandwidth of the recorded luminance signal. Then, the jitter included in the video signal played back from the first VCR would have to be corrected before performing three-dimensional signal processing--i.e., processing both in the one-dimensional time domain and in the two-dimensional spatial domain--as a procedure in the recording with the second VCR.
In professional video recorders, such as those used in television broadcasting, the tape spooling and capstan mechanisms are considerably better than home VCRs in regard to avoiding undesired tape movements. The results of the recording procedure can be monitored by separate playback heads. The operating speed of the headwheel is controlled by a servomechanism that corrects by degenerative feedback the frequency and phase errors between a very stable, crystal local color oscillator and the color burst regenerated from played-back video. The reduction of TBE during recording makes it substantially easier to time-base-correct the video signal during playback. The foregoing measures are too expensive to be used in home VCRs.
In prior-art home VCRs time-base correction is carried out only during playback and without involving the use of servomechanisms to finely regulate operating speeds. The complexity of the TBE correcting circuits that would be required to provide time stability between successive fields of video samples is increased by the fact that the signal processing in the two-dimensional spatial domain is done over a region spanning seven lines in the vertical direction. That is, the problem of implementing TBC is compounded by the need simultaneously to align samples in adjoining scan lines so as to implement signal processing in the two-dimensional spatial domain. Accordingly, the time-base stability of the corrected signal from the TBC is insufficient for performing high-resolution processing in the temporal domain as well as the two-dimensional spatial domain. The phase-locked oscillator (PLO) circuit which adjusts the timing of the TBC input sampling clock so as to synchronize with a multiple of the horizontal sync signal frequency, is unable to track the error of the input signal sufficiently closely, particularly during the time that head switching takes place and timing errors tend to be most severe. Tracking problems arise because of noise; because the time constant of the corrective-feedback loop connection that is used to control the frequency and phase of the oscillator has to be too slow, so as not to disrupt spatial filtering procedures; or because of feedback loop instability during playback.